The present invention relates to a power margin indicator for a rotary wing aircraft, particularly a helicopter.
A power margin indicator such as this is intended to provide information about the power margin available on at least one engine and one gearbox, generally the main gearbox, of said aircraft.
Patent FR-2 756 256 in the name of the applicant discloses an indicator of this type which comprises:
acquisition means which determine the values of parameters for the control of the engine and the gearbox;
calculation means which, on the basis of said values of the control parameters received from said acquisition means and of limit values regarding these control parameters for various engine speeds, determine for each of said speeds a power margin, each of said power margins being determined for the corresponding engine speed with respect to a first limit which corresponds to the first of the limit values which would be reached by one of the control parameters if the power were to vary; and
display means which display said first limits on a display screen for at least some of said speeds, which first limits are represented on a scale graduated in collective pitch equivalents and capable of scrolling past a fixed pointer which indicates the effective collective pitch, the differents in terms of collective pitch between said pointer and one of said first limits being representative of the corresponding power margin.
This known indicator therefore makes it possible, at any moment and as a synthetic overview, to display:
the effective (current) value of the collective pitch; and
the values of the first limits for each of the engine speeds considered.
It will be noted that, for an AEO (All Engines Operative) flight configuration in which all the aircraft engines are operative, this known indicator is in itself sufficient to allow a pilot to control the engine and the main gearbox with respect to the aforementioned limit values.
However, this known indicator is not sufficient when an aircraft comprising at least two engines is in an OEI (One Engine Inoperative) flight configuration in which at least one engine of said aircraft has failed.
What happens in this case is that as long as a conventional limit (known as the FADEC limit) defined in the known way by an engine control system of the conventional FADEC type is not reached, the power increases and the value of the current pitch (represented by the aforementioned fixed pointer) converges toward said limitation as the pilot increases the collective pitch. The rotor speed is therefore set to a nominal speed or set point speed via said engine control system. By contrast, as soon as said limit is reached, the rotor speed drops as the collective pitch increases. The known and aforementioned power margin indicator then no longer allows the aircraft to be controlled and a specific NR (rotor speed) indicator is then necessary.
In consequence, in the OEI flight configuration, two different indicators (the power margin indicator and the NR indicator) are needed to allow control. This of course presents drawbacks, in particular:
significant bulk;
reduced comfort for the pilot(s), because two indicators have to be monitored; and
the need for the pilot(s) to pay extra attention, particularly in order to determine which of these two indicators is operative.
The power margin indicator disclosed by patent FR-2 756 256 is therefore unable, by itself, to allow control for all flight configurations.
The present invention relates to a power margin indicator which allows the aforementioned drawbacks to be overcome.
To this end, according to the invention, said indicator of the type comprising:
acquisition means which determine the values of parameters for the control of the engine and the gearbox;
calculation means which, on the basis of said values of the control parameters received from said acquisition means and of limit values regarding these control parameters for various engine speeds, determine for each of said speeds a power margin, each of said power margins being determined for the corresponding engine speed with respect to a first limit which corresponds to the first of the limit values which would be reached by one of the control parameters if the power were to vary; and
display means which display said first limits on a display screen for at least some of said speeds, which first limits are represented on a scale graduated in collective pitch equivalents and capable of scrolling past a fixed pointer which indicates the current collective pitch, the difference in terms of collective pitch between said pointer and one of said first limits being representative of the corresponding power margin,
is noteworthy in that said acquisition means also determine the values of said rotor speed, and in that said calculation means take account of said rotor speed values and of at least one rotor speed set point value to determine, regardless of the flight configuration, at least one power margin which is such that when the rotor speed drops below said set point value the corresponding first limit which is displayed by the display means indicates to the pilot of the aircraft that he should reduce the collective pitch until the rotor speed returns to said set point value.
Thus, by virtue of the invention, said power margin indicator, while having the advantages of the known and aforementioned indicator, allows the pilot to keep the rotor speed at a set point value and to do so regardless of the flight configuration, that is to say to do so both in an AEO configuration and in an OEI configuration.
In consequence, this indicator according to the invention is sufficient, in itself, to allow control in all flight configurations considered, which makes it possible to remedy the aforementioned drawbacks.
According to the invention, said rotor speed set point value which preferably corresponds to the value recommended by the aircraft flight manual:
depends on the current flight configuration; and/or
represents a percentage of a customary reference value which, in the known way, depends on flight parameters such as the pressure P0, temperature T0 and speed Vi of the aircraft.
By way of illustration:
in the AEO flight configuration, said set point value preferably represents 100% of the reference value (NR 100%); and
in the OEI flight configuration, said set point value represents, for example, between 95% and 100%, preferably 97%, of the reference value (NR 97%).
Furthermore, in one preferred embodiment, said acquisition means comprise a conventional phonic wheel mounted on said rotor to measure the value of said rotor speed. Of course, other known measurement means may also be used.
Incidentally, advantageously, for an AEO flight configuration, for which all the aircraft engines are operative, said calculation means determine and said display means display the power margins relating at least to:
the maximum transient power;
the maximum take-off power; and
the maximum continuous power, all specified hereinabove.
In addition, advantageously, said calculation means determine and said display means display, in addition, a power margin which is such that the corresponding first limit represents a limit value for if one engine of the aircraft fails.
This makes it possible, during the AEO flight configuration, to obtain precise information about any later OEI flight configuration there might be so that, if an engine should fail, there is no imprecise transient phase on the display when changing from the current AEO flight configuration to an OEI flight configuration.
Furthermore, advantageously, said display means display a single marking for the torque which illustrates both the maximum take-off power and the maximum continuous power and which depends on the aircraft flight speed. The continuous limit thus obtained makes it possible to allow steady state with limited torque with no limit on time. This also allows better protection for the main gearbox by applying more realistic limits.
Incidentally, according to the invention, for an OEI flight configuration for which at least one of the aircraft engines is inoperative, said indicator is capable of indicating power margins of at least one of the following two modes:
a first mode for which the limitation is defined with respect to the super-contingency power specified hereinbelow; and
a second mode for which the limitation is defined with respect to the maximum contingency power specified hereinbelow.
To do this, the indicator according to the invention additionally comprises:
automatic means for triggering said first mode when an engine failure occurs; and
manual means capable of being actuated by a pilot to switch from said first mode to said second mode and vice versa.
Advantageously, for said first mode, said calculation means determine and said display means display the power margins relating at least to:
the super-contingency power with the rotor speed at the set point value for the OEI flight configuration;
the maximum contingency power with the rotor speed at the set point value for the AEO flight configuration; and
the intermediate contingency power specified hereinbelow with the rotor speed at the set point value for the AEO flight configuration; and possibly
the maximum contingency power with the rotor speed at the set point value for the OEI flight configuration.
In addition, advantageously, for said second mode, said calculation means determine and said display means display the power margins relating at least to:
the super-contingency power with the rotor speed at the set point value for the OEI flight configuration;
the maximum contingency power with the rotor speed at the set point value for the OEI flight configuration; and
the intermediate contingency power with the rotor speed at the set point value for the AEO flight configuration.